Refactor MapperOutput.__getitem__
Refactor MapperOutput.__getitem__

Create getitem_composite and use that function to check the two dicts
(self.data and self.computed) for the given key.

This change was made to make it easier for subclasses to override
__getitem__ with their own sets of dictionaries to check.

wcmitchell/insights-core,PaulWay/insights-core,PaulWay/insights-core,wcmitchell/insights-core